The present invention relates to the destruction of hetero-atom organics using transition-alkaline-rare earth metal alloys and, more particularly, to a method and system for decomposing or immobilizing organic wastes using metal alloys which have been decrepitated and activated by exposure to oxygen and hydrogen.
Oxides of alkaline metals are known to be effective in the destruction of a wide spectrum of waste chemicals including halogenated, sulfur-containing, and phosphorous-containing compounds. Compounds which contain double or triple bonds are also generally more reactive, and can also be decomposed or immobilized (e.g. adsorbed) by oxide materials. In particular, microscale and nanoscale metal oxides have been demonstrated to destroy compounds with byproducts of formic acid, CO.sub.2, water, CO, and metal salts.
Several distinct preparation methods and final physical forms for these organic waste destruction materials are known in the art, including aerogels, high vacuum thermal activation, and laser vaporization/condensation. These oxides, which use an adsorption mechanism to decomposition organic compounds, can develop an "ash layer", presumed to be comprised of reaction products, through which the reacting species must diffuse prior to reaction with the underlying fresh substrate. Consequently, these metal oxides have the disadvantage of tending to develop a non-reacting barrier which slows or stops the organic compound decomposition process.
Metal hydrides are also used in organic chemical synthesis as a source of hydrogen. There are also commercial processes which use a separate catalyst and hydrogen gas as one reactant (e.g., polymerization, catalytic reformers).
Catalysts are known in the art to facilitate chemical reformation. By definition, a catalyst is a compound that does not directly participate in the reaction scheme of chemical process, but rather, the catalyst decreases the activation energy for the chemical process so that it may proceed at a more rapid pace or to higher conversion efficiencies. Generally, catalysts are broken down into three categories. The first category is metal conductors consisting of transition and precious metals such as Fe, Pt, Pd, and Ag can chemisorb oxygen and hydrogen and are generally used in hydrogenation and dehydrogenation reactions. The second category is insulators consisting of metal oxides such as Al.sub.2 O.sub.3 and MgO, which are generally considered to be acidic, and are generally used in cracking, polymerization, alkylation, isomerization, and hydration-dehydration reactions. The third category is semiconductors, consisting of compounds such as NiO, ZnO, TiO.sub.2, and V.sub.2 O.sub.5. The catalytic capabilities of the semiconductor catalysts are well-known. Replacements for precious metal catalysts are also being developed, some comprised of nanoscale powders including iron, iron sulfide, and molybdenum disulfide.
Useful catalysts are also comprised of physical mixtures or combinations of the above-mentioned catalyst categories. For example, a physical mixture of MgO and Ni is used for steam-methane reactions, Ag and Al.sub.2 O.sub.3 is used as an ethylene oxide catalyst, Zn.sub.5 Cu alloy catalysts is used for methanol synthesis, and Cu--Ni alloys are used for ethane hydrogenolysis. The use of catalytic metal oxides MoO.sub.3 --Al.sub.2 O.sub.3 in physical combination with an alkali (Group IA) or alkaline (Group IIA) metal hydroxide is already known. Catalysts of many other transition metal combinations or transition metal-transition metal oxides have also been described in literature including Pd--Pt, Ru--Fe, Cu--ZnO, Fe--Cu, Cu--Co, Bi--Pt, Pd--Cu, Zn--Ru, Rh--Mo, Ni--Ru, and ZrO.sub.2 --CuO. The oxidation of chlorinated organics using physical mixtures of transition metal oxides, alkaline metal sulfates, and precious metals is also known.
The physical combination of Fe.sub.2 O.sub.3 catalysts with CaO has also been suggested for the application of destructive adsorbents whereby the CaO is coated with a layer of Fe.sub.2 O.sub.3. Coating or depositing a layer of catalytic transition metal oxide onto the surface of alkaline metal oxide has been met, however, with difficulty.
An object of the present invention is to provide an improved method for destroying hetero-atom organic compounds containing halogens and/or sulfur and/or phosphorous, and/or single, double or triple bonds by using a complex metal alloy comprised of one or more transition metals with one or more alkaline metal and/or one or more rare earth metal. We have found that a combination of these metals in the form of a metal alloy solid solution can improve the destructive potential over those produced by the metal components separately.
Another object of the present invention is to utilize the constituents of multi-component catalysts or destructive substrates in a combined chemical solid solution form, i.e. an alloy, rather than physical mixtures or coatings. The present invention is particularly advantageous in this regard because metallic alloy solutions place the components within atomic distances throughout the entire metal material, whereas coatings have specific two-dimensional contact points and physical mixtures have the metals separated by distances similar to the particle size.
The present invention has the substantial advantage over known methods in which physical combinations of the elements require complicated formulations, difficult deposition processes, considerable process control to insure homogenous mixing, and precautions to avoid mutual chemical reactions such as poisoning or consumption of one component by another.
The present invention recognizes that catalysts and destructive adsorbent metals and metal oxides, combined in an alloy form and activated in a system according to the present invention, will decompose organic molecules comprising hetero-atoms more efficiently than the constituents acting separately. This approach differs from conventional methods in that the synergy of using the constituents in chemical solutions or in an alloy has not been used for catalysis or destructive adsorption substrates.
The present invention also recognizes that metal catalysts and metal hydrides, combined in an alloy form and activated in the system according to the present invention, will react with hetero-atom containing organic species without the need for externally supplying the hydrogen gas and providing a separate catalyst bed for the reaction. This approach differs from the conventional industry approach because separate catalysts are typically employed and hydrogen is fed to the reactor as a gas.
Another object of the present invention is to provide a preparation method and system using hydrogen and oxygen cycling which decrepitates, exposes, and activates the metal alloy surface.
Still another object of the present invention is to enhance the decrepitation and activation process by milling the metal alloy while in contact with hydrogen or oxygen or compressed air.
The preparation of fine scale powders generally falls within known chemical or mechanical production methods. Chemically formed powders include aerogels, precipitants, chemical reactions, and vapor deposition. Mechanical methods rely on milling, crushing, or exploding. Many of these methods for powder production are utilized in the field of metal parts fabrication or ceramics. Mechanical decrepitation of the metal hydride compounds is known for the preparation of fine powders of the hydride or base metal and makes use of the embrittlement induced by hydride phase formation in metals, such as, for example, hydriding zirconium to promote embrittlement for further machine working. Similarly, titanium hydrides have been hydrided, crushed, molded and sintered to produce metal parts (see, for example, Uenishi Japanese patent document 63089636), and niobium hydrides were thermally cycled to decrepitate metal.
Metal powders which reversibly form metal hydrides can also be decrepitated by hydride-dehydride cycling as described by U.S. Pat. No. 4,893,756. There the apparatus and process for hydride-dehydride metal hydride cycling is provided for the purpose of comminuting an ingot of metal hydride for hydrogen storage applications, specifically for use in electrochemical cells. That document does not demonstrate, however, the advantages of decrepitation and embrittlement resulting from alternating oxygen and hydrogen exposures, i.e. hydrogen/oxygen cycling, nor is the added benefit of performing oxygen/hydrogen cycling in conjunction with mechanical milling recognized.
The method of the present invention utilizes a novel combination of chemical and mechanical particle decrepitation, namely, the reaction of oxygen and hydrogen within the metallic lattice and mechanical disintegration aided by hydrogen embrittlement. That is, the present invention uses hydriding and oxidizing of metals which forms H.sub.2 O within the metal lattice thereby causing local distortions and dislocations (defects) of the metal lattice. In cycling or exposing an oxide or hydride to hydrogen or oxygen, respectively, water is formed at grain boundary sites on the metal accompanied by vast increases in volume, and therefore stresses. This stress is typically relieved by generation of cracks and holes in the lattice. In addition to the effect of water formation, the hydride cycling process alone generates lattice expansions because the density of the metal hydride is less than the pure metal. This process also generates internal stress, relieved through the formation of cracks and holes. The use of mechanical milling or agitation has also been found to facilitate stress relief in hydride cycling and hydride and oxide cycling.
Although embrittlement by hydrogen to facilitate mechanical decrepitation and the method of hydride-dehydride cycling to decrepitate metals and metal alloys which form hydrides are known, we were the first to discover the benefit of alternating hydrogen-oxygen (or air) exposures. The use of oxygen exposure as an integral step in powder decrepitation is counter-intuitive because the exposure of the hydriding-dehydriding material to oxygen has generally been avoided given the fact that oxygen is a known poison to reversible metal hydrides in a setting of their intended use.
The present invention teaches for the first time that the use of oxygen cycling in conjunction with hydrogen cycling can in fact be beneficial for powder decrepitation due to the large internal stresses generated by the formation of water molecules within the structure of the metal alloy where previously only an oxide or hydride specie was present.
Furthermore, the present invention departs from the prior art which did not use mechanical agitation of the material during the oxygen/hydrogen cycling to further enhance the decrepitation of the metal alloy resulting from embrittlement and large internal stresses caused by the presence of water molecules within the metal alloy structure.
A presently preferred method for decomposing and/or immobilizing organic wastes which may contain halogens, sulfur, phosphorous, single bonds, double bonds, and triple bonds comprises the step of first preparing complex metal alloys through decrepitation and activation using a cyclic hydrogen, oxygen or air exposure process; and secondly, contacting the activated complex metal alloy with the organic waste compound. The complex metal alloys are comprised of two or more metals comprised of transition metals, alkaline metals (Group IIA) and/or one or more rare earth metals, and prepared using a process which decrepitates and creates active metal, metal oxide and/or metal hydride surface.
The activation process in accordance with the present invention comprises the step of exposing the metal alloy to hydrogen and oxygen or air to activate the surface and decrepitate the powder to a form with a higher surface area and higher lattice defect. The activation process can also be enhanced or accelerated further by milling the metal alloy while exposing it to the hydrogen so the material is in a hydrided state, or by milling the metal alloy while exposing it to oxygen or air after exposure to hydrogen, thereby leaving hydrogen absorbed on the metal alloy thereby producing an alloy comprised of catalyst and hydrogen source.
We have found that milling the materials in the presence of hydrogen or oxygen accelerates the reaction of the metal alloys with the oxygen or hydrogen by exposing fresh surface through which the oxygen or hydrogen can diffuse and react with the underlying metal alloy thereby forming the oxide or hydride materials. The milling process also relieves stresses accumulated within the lattice of the metal alloy due to the presence and volume differential associated with oxides, hydrides, and water.
The mechanical milling process can be carried out with a standard ball mill or milling jar with grinding media such as burundum, zirconia, nylon, or polyurethane grinding stones. Ultrasonic agitation is also another contemplated effective method to mechanically agitate or mill the particulates when oxides, hydrides, or water are present in the metallic lattice. The milling operation takes place according to a prescribed end point particle size established by prior testing using known techniques.
Regardless of the previous number of hydrogen and oxygen exposure cycles, the hydrogen/oxygen exposure process terminates, with an exposure to oxygen or air, to form an active surface oxide. The powdered metal alloy is then placed in contact with the waste organic compound.
The complex metal alloys utilized in the present invention have at least one metal which forms a stable oxide and one component which is generally described as a catalyst. For example, the alloys CaNi.sub.5, Mg.sub.2 Ni, and LaNi.sub.5, Ca, Mg, and La form very stable oxides, and the presence of Ni or NiO on the surface of these alloys serves as a catalyst. Rare earth metals other than La are contemplated as equally effective in alloy compositions. For example, a combination of rare earth metals termed "mischmetal" which includes La, Ce, Nd, and Pr is sometimes used in place of La. In the alloys TiFe or TiFe.sub.0.9 Mn.sub.0.1, a combination of two or more transition metals, the components may form an oxide. In this case, the iron, titanium and manganese may serve as oxidation agents or catalysts.
We attribute the effectiveness of the complex alloys of our invention to a synergism between the metallic components which has not heretofore been achieved with a metal oxide destructive adsorbent or transition metal catalyst separately. Likewise a physical mixture or coating cannot achieve the same properties as the present invention because we believe that the metal components are not within atomic distances of one another throughout the entire bulk of the compound.